Hero999

What level? Primary School? High School? Special School?

Unfortunately that's not possible. All posts before 24th January 2009 have been lost due to an error in the database. http://www.electronics-lab.com/forum/index.php?action=profile;u=34329;sa=showPosts;start=392 Previously all posts were replaced with symbols but they've notw changed to an error message. http://www.electronics-lab.com/forum/index.php?topic=18829.msg86940#msg86940

This is a silly question as fars as I'm concerned. I don't have a favourite component, I just use what gets the job done in a particular application. If I want a stable low current 5V power supply I'll use a regulator such as the LM7805, if I want to siwtch a motor on and off I'll use a power MOSFET and if I need to illuminate something I'll use an LED.

It's probably a SMPS so this is pretty normal. The power supply has a maximum power rating, this means that at higher voltages the maximum current will be lower than at lower voltages. The maximum power rating should be listed somewhere in the specification. Having a maximum power rating rather than just a maximum current rating is a good thing: it makes the power supply more flexible. Your power supply has a maximum power rating of 18W, if they decided to have a fixed current limit then it would only be capable of 0.6A at any voltage. If I was given the choice between this PSU and an SLA of the same price I'd go for the PSU for general purpose experimenting. If I just wanted to power a motor then I'd go for the SLA. I haven't checked out the prices of lab power supplies recently but a 180W power supply won't be cheap. For higher current applications I recommend buying a 12V or 24V fixed supply which will give a better price per Watt. Lab supplies aren't really intended for powering large motors, they're made for electronic circuits.

For the third and final time a jammer will only make it worse.

It depends on the PWM circuit. The resistance shouldn't be a problem since the potentiometer's resistance will dominate but inductance and capacitance (if the potentiometer forms part of the oscillator) or noise pick-up could be a problem. Do you have the schematic? Noise can be reduced by twisting the wires together. Does the potentiometer just vary the voltage? If so inductance and capacitance would be a problem. The noise can be mitigated by twisting the wires together and connecting 100nF capacitors from either side of the pot to the wiper at the PWM circuit side of the cable.

What are you trying to do? Naughty, naughty you shouldn't be spying on the girl's changing room. ::)

Why not? If another magnet in phase with the speaker magnet makes the field stronger then another magnet out of phase the speaker magnet will make it weaker. Surely a stronger field means higher sensitivity and a weaker field means lower sensitivity.

I don't see why not.

That sounds a bit complicated for something they stick on your car.

Yes, it should be R2.

Doesn't that lower the sensitivity?

I doubt they're continuously bribing the police, many wealthy people have been arrested, put on trial and sentenced. The idea that they've somehow corrupted the local authorities doesn't make sense, of course bribery and corruption do occur but there's a difference between bribing the odd official and the whole local government. Do you live in a developing country? If so I might believe you but this sort of thing doesn't happen on such a large scale in more wealthy countries such as the US. The people at the top already get paid enough and in general have too much to loose from being bribed. As I said above I'm not saying it doesn't happen, just not on the scale you're talking about. That's a more viable option. If you knew the frequency then you could easily build something, the trouble it you don't so you're best bet is to buy a radio scanner to find out, even then it might not be easy if they're using something fancy such as frequency hopping.

A typical opto-TRIAC can only take a couple of hundred mA which isn't enough for an air conditioner. It's up to you, you could use a transistor and relay if you like. All I'm saying is that it's wasteful to use both an opto-isolator and a relay.

Have you done anything to upset someone with a lot of money? Be honest, you haven't sold anyone poor quality drugs have you? If you've ripped off a gang then be prepared to pay.

That will work, assuming the opto-isolator is rated to carry the relay current. It seems a bit overkill to use both an opto-isolator and a relay. I would have used either an opto-TRIAC triggering a TRIAC or a transistor and relay. Relays are more fail safe than TRIACs but have the disadvantage of requiring a power supply. An opto TRIAC triggering a TRIAC can take all the power it requires from the parallel port.

Aren't engines limited anyway? Unfortunately they're normally set at the start or the red line but you might be able to adjust it to a more sane maximum speed. Alternatively you could use an optical tachometer. Are you using a microcontroller? If so it should be easy to count the pulses. If not there are other ways, build a counter with digital logic to count the pulses, use an oscillator to reset the counter at a regular intervals. Use a digital comparator or logic gates to monitor the counter, if it exceeds a certain count the engine could be cut-off or its speed reduced. For example if the tachometer gives one pulse per revolution and you reset the counter every second, if the count exceeds 50 which is 3000 rpm cut-off the engine. You'll need to add a Schmitt trigger to get a square waveform whichever method you choose to count the pulses. http://www.vk2zay.net/article/29

Take a closer look at the filter capacitors: Are any of them swollen? Is there any sign of fluid leaking out of them? Are the terminals corroded? If the answer to all of the above is no, they might all right, although filter capacitors do go bad without showing any physical sings of doing so. The risk yo take is a bad filter capacitor taking out the MOSFETs.

Why not just increase the speed when the battery voltage drops below a certain level?

What's a PI? It sounds like you're just being paranoid. The only secret police in the US are the FBI and they have restricted powers. Why don't you emigrate to Russia or China where the secret police can beat you up and make you mysteriously disappear?

It's an NTC (negative temperature coefficient) resistor. When it's cold it has a fairly high resistance, as it heats up its resistance drops. It's used a an inrush current limiter. When the power is first applied it's cold and its high resistance limits the current, as it warms up its resistance drops, allowing the full current to flow. It's needed because the filter capacitor across the rectifier on the mains inlet draws a huge current when the power is applied. The chances are other components have failed, I recommend checking the rectifier and filter capacitor (both value and ESR), if you don't have an ESR meter (value alone isnt' good enough) then replace the capacitor as a matter of course. Failure of the filter capacitor, rectifier or chopper MOSFET(s) could cause the NTC resistor to fail. Additionally failure of the filter capacitor can cause the rectifier and MOSFET(s) to fail and failure of the MOSFET(s) can cause the rectifier and associated components to fail. Because of this, it can be pretty hard to find out what went wrong first. There's also a good chance many other components have failed rendering it beyond economical repair.

Remove the tracker device. Why is the tracker there anyway? You say you're being stalked, report it to the police. Do you live in a country with an authoritarian government? Perhaps you're being spied on by the secret police. Just inspect your car for tracking devices before you drive.

The same way they can track the tracker. They use a direction finder and field strength meter to get a rough direction and distance between them and you. If anything, the jammer is going to make their life easier not harder as the signal from your car will be stronger.

Because that's what inductors do: they try to keep the current constant. Think of a flywheel: difficult to get spinning and hard to stop once spinning. An inductor stores energy in the form of a magnetic field which takes energy to build and releases energy when it collapses. When there's no current flowing through the coil and its energised, energy is needed to build the field. If the current is suddenly interrupted, the field collapses which induces a high voltage spike which forces the current to carry on flowing by arcing over the switch contact.

What's an SPS relay? Single Pole Single What? Throw, I'd guess? No conversion is necessary, just only use one of the poles or better, connect them in parallel to increase relay life.